It is well known that for transmission of a pictorial digital image over a narrow band communication channel, block transform coding techniques (such as discrete cosine transform DCT) achieve very high bit rate compression ratios. Spatial transform coding schemes are able to achieve such high bit rate compression ratios due to the fact that the spatial transformation concentrates most of the energy of a pictorial image in the transform coefficients representing lower spatial frequencies. When the transform coefficients are arranged in order of increasing spatial frequency, for example by the well known zig-zag scan technique, long runs of zero amplitude coefficients are generated. The values of the transform coefficients are Huffman encoded, and the strings of zero amplitude coefficients are run-length encoded for very efficient image compression. See U.S. Pat. No. 4,302,775 issued Nov. 24, 1981 to Widergren et al for an example of such a compression scheme in a video image compression system.
Using a block transform coding scheme, it is possible to transmit a full resolution television frame over a telephone line in less than one minute. It has also been suggested (see the article "Image display techniques using the cosine transform" by King N. Ngan, IEEE transactions on acoustics and signal processing, Vol. ASSP-32 No. 1, February 1984.) that in a low bit rate image transmission system it would be desirable to provide the viewer at the receiver with a quick (within several seconds) "preview" image during transmission of the full resolution image, so that the viewer can quickly evaluate the usefulness of the image being transmitted, and terminate transmission if the full resolution image is not of interest, thereby effecting savings in time and cost. This capability would be very useful for example to quickly browse through a number of images (e.g..about.100) to find a particular image that is desired. Ngan suggests that such a low resolution preview image can be developed from the first few transform coefficients of an image by transmitting the transform coefficients in an order representing increasing spatial frequencies, and performing an inverse transformation on the first few coefficients that are received to provide a preview image.
In a practical pictorial image transmission system, the scheme suggested by Ngan for providing a preview image has several disadvantages which were not discussed in Ngan's article. In a block transform image compression scheme, the image is divided into blocks (e.g. 16.times.16) of pixels. Consecutive blocks of the image are transformed and the coded transform coefficients are transmitted. The progressive transmission scheme suggested by Ngan, wherein only the first few coefficients from each block are used to generate a preview image, was found by the present inventors to be impractical in an actual image transmission system, since transmitting only the first few coefficients of each block frustrates the run-length coding of the coefficients. Furthermore, since all the coefficients are needed to recover the full resolution image at the receiver, the lower frequency transform coefficients from all the blocks must be stored until the higher frequency coefficients are received, thereby increasing the memory requirements at the receiver.
It is the object of the present invention to provide a system for transmitting pictorial still images over a narrow band transmission channel such as a telephone line, including means for presenting a low resolution preview image that is free of the disadvantages noted above. It is a further object to provide such a system for transmitting a color pictorial image.